1. Field of the Invention
The present invention relates to refrigerator and/or freezer units for perishables and more particularly to such units having an alarm system for preventing loss of perishables by signalling when an access door is not closed.
2. Description of the Related Art
Perishable materials of various sorts are kept in refrigerator and/or freezer units in order to maximize shelf life. An example is the household refrigerator-freezer unit in which substantial amounts of refrigerated foodstuffs may be kept in separate compartments. If the access door of an unattended refrigerator-freezer unit is not fully closed, the open compartment can become sufficiently warm that the contents are damaged by defrosting, spoilage or both. If the access door remains open long enough without being discovered the unit itself can be damaged, usually as a result of wear and tear on the refrigerant compressor.
The same problems are encountered in freezers and single door refrigerators. For convenience, all these units are identified collectively as "refrigeration units."
It has been recognized that door ajar problems can be minimized by drawing attention to the unclosed access door before any harm occurs. Alarms for signalling when an access door is not fully closed have been proposed. An alarm capable of effectively signalling an open door condition can be annoying to a user who keeps the door open during loading the refrigeration units, cleaning or defrosting. Alarm systems have thus been proposed for signalling a door open condition without producing an alarm when the user intends leaving the door open for an extended period. U.S. Pat. No. 3,996,434 discloses one such approach.
Other prior art proposals have been constructed to sense when an access door is open and produce an alarm after a predetermined time. These avoided undesired alarms when the unit was being loaded or unloaded, yet produced a warning when an access door was open overlong.
So called "after market" alarms, constructed and arranged for attachment to an existing unit by a serviceman or its owner, have also been proposed. These alarms were principally interesting to those who suffered losses from an open access door and were willing to spend money to avoid a recurrence. Such alarm units were designed and constructed for "universal" application to many different kinds and types of units. These alarms were expensive, because relatively few were produced, unattractive when installed and not particularly effective when used with certain units. An example of an after market type of alarm is disclosed in U.S. Pat. No. 4,691,195.
Alarms proposed for assembly in refrigeration units by the original equipment manufacturer (OEM) had the advantage of being "built in" and therefore not unattractive. These alarms, because constructed and arranged specially for particular units, tended to operate more reliably with their units than did after market alarms placed on the same units. O.E.M. installed alarms were complex (and thus expensive), so that their inclusion in production refrigeration units added materially to the retail selling price. Worse, they were not always reliable. Alarms which failed to detect open access doors were just as unacceptable as alarms which falsely signalled an open door condition. In the latter case a service call would be required to correct the problem.
Some prior art proposals utilized access door position sensors formed by electric switches for controlling piezo electric alarm devices. U.S. Pat. No. 4,707,684 discloses a door ajar sensor arrangement having a door actuated control switch coupled to a piezo electric sound transducer through a timer. When the door closed a switch plunger was depressed and the alarm disabled. When the door opened, the plunger was positioned to operate the timer. When the timer timed out, the transducer was energized and sounded the alarm.
This seemingly straight forward approach has not been favored because plunger type switches, in the environment of a refrigeration unit door ajar sensor, are not reliable over the typical life of such a unit. The switches must be positioned remote from the door hinges in order to assure sensitivity to a door ajar condition. When placed at such locations the plungers are apt to become fouled as a result of food spills and jammed in the depressed position. When struck by objects as the unit was being loaded or unloaded, the plungers were sometimes deformed and jammed in their depressed positions. When a plunger jammed in the depressed position the alarm was disabled.
This failure mode was not "fail-safe" because access doors could remain ajar without any alarm being produced. Furthermore, there was little likelihood the switch failure would be noticed before a loss was experienced.
Magnetically operated sensors have been proposed to avoid such problems. These sensors can be constructed as sealed units disposed within the cabinet walls or an access door and thus not subject to failure as a result of impacts or food spills. One proposal of this general type is disclosed by U.S. Pat. No. 4,241,337 wherein access doors of a side-by-side refrigerator-freezer are provided with a magnet and a sensor unit, respectively.
In this proposal the sensor was formed by Hall effect switches in one access door. Fully closing both access doors precisely aligned the sensor with a magnet carried by the other access door. The Hall effect switches changed conductive states when the doors fully closed so that no door ajar alarm sounded. If the doors are not fully closed for a predetermined time an alarm sounded.
These kinds of alarms had relatively many parts and required special construction of the refrigerator-freezer units containing them. Furthermore the doors on such units tended to droop after a period of use which led to magnet and sensor misalignment. Misalignments created false door ajar alarms and led to otherwise unnecessary service calls. Still further, the door magnets are so weak that amplification of the Hall effect sensor outputs is often required in order to produce a usable signal.
Refrigerator and freezer doors commonly carry peripheral sealing gaskets containing "latching" magnets for securing the door closed. The gaskets are rubberlike and tubular. The magnets are long flexible strips placed inside the gaskets. When a door is closed the magnet is coupled to the magnetic cabinet material extending about the access opening.
The magnets typically extend throughout the length of the gasket and are intentionally made to be "weak." This minimizes the force required to open the door and is an important safety feature. In the past these magnets have been used to operate magnetically sensitive elements to govern a function associated with the refrigerator. An example is disclosed by U.S. Pat. No. 2,957,320 where the gasket magnet is used to operate a compartment light.
Use of door gasket magnets in door ajar alarms would seem an attractive idea because the alarm cost might be reduced significantly. There were some serious practical drawbacks. Door gasket magnets were not precisely located from unit to unit. First, the gasket locations on the doors ranged within relatively wide tolerance bands. Thus the magnet locations were not tightly controlled. Second, since access doors sag over time, as noted above, the gasket and magnet positions relative to the refrigeration unit were not predictable during use. This combination of factors made accurate detection of door position difficult.
Aggravating the situation is the fact that the door gasket magnets produce weak, peculiarly shaped magnetic fields. The magnets are typically elongated strips constructed with a central longitudinally extending south pole and north poles extending along the opposite lateral sides of the strips. The fields produced by such magnets are quite narrow.
Accurately determining door position for door ajar alarm purposes is thus a difficult matter compared to merely sensing whether a compartment light might be turned off. In the former case the alarm system must be capable of accurately discriminating between an indicated door ajar condition and a door closed condition indicated by a narrow, weak magnetic field produced by a magnet having a position which can vary relatively widely from unit to unit.
U.S. Pat. No. 4,891,626 proposes constructing a door ajar alarm system employing multiple magnetic field responsive switches for detecting a single magnetic door gasket. The switches are mounted in the compartment wall surrounding the access opening. They are offset from each other relative to the nominal magnet location so magnet position variations from unit to unit do not prevent the alarm from operating properly. The switches are connected in parallel with each other so actuation of any one of them serves to enable the alarm system.
The '626 patent proposes reed switches or hall effect switches mounted along a line extending at an angle through the narrow elongated magnetic field produced by the door gasket magnet. The respective switch axes are oriented parallel to each other while the switches are offset laterally relative to the magnetic field location. This arrangement of two or more switches is employed to assure that the alarm system is responds regardless of gasket magnet location variations. The use of plural duplicate switches is costly and unduly complicates the alarm system construction. The multiplicity of parts and circuits also tends toward less reliable operation.
The magnetically responsive switches are connected to a remote alarm circuit mounted in the appliance so that when the door is not fully closed the alarm is tripped by one or more of the switches. The alarm system employs a commercially constructed buzzer device and circuit for operating the buzzer. Alternative alarm systems used commercially available piezoelectric alarm devices with operating circuits as noted previously.
These kinds of alarm devices were usually mounted in a separate housing attached to the refrigeration unit. They were expensive because they employed purchased, or separately constructed, components and subassemblies which had to be assembled into a housing and mounted to the appliance.
The alarm housings were generally placed on the exterior of the appliance to maximize their loudness. The environment in which the unit was placed was often such that the alarm units could be fouled by dust, dirt and airborne cooking oil elements common in kitchens. In some instances the units were exposed to extremes of heat and cold, for example when used with a freezer located in an unheated basement or porch.
The present invention provides a new and improved door ajar alarm system for a refrigeration unit providing a simplified, highly effective sensor for detecting when a door is not fully closed and producing an alarm.